Following a meal, hepatic parasympathetic nerves provide a permissive signal to the liver that regulates the ability of insulin to stimulate the release of a hormone, HISS, from the liver. HISS selectively stimulates glucose uptake and storage as glycogen in skeletal muscle and accounts for over one-half of the whole body glucose disposal that has previously been assumed to be a direct effect of insulin. Hepatic sympathetic nerves block the parasympathetic signal thus preventing the release of HISS and resulting in a 50% reduction in the glucose disposal effect of insulin. This condition is referred to as HISS-dependent insulin resistance (HDIR).
HISS action can be clinically diagnosed by determining the response to insulin in the fasted state and following re-feeding. The difference in the glucose disposal effect of an injection of insulin determined in the fed and fasted state represents the HISS-dependent component of insulin action. The glucose disposal produced in the fasted state is independent of HISS whereas the approximately doubled effect of insulin following a meal is due to both the HISS-dependent and HISS-independent component of insulin action with the difference between the two states being defined as the HISS-dependent component of insulin action.
HISS-dependent and HISS-independent insulin action can be most readily quantitated using the rapid insulin sensitivity test (RIST) which is a transient euglycemic clamp in response to a bolus administration of insulin. Normally insulin injection stimulates removal of glucose from the blood into storage sites with a resultant decrease in blood glucose level occurring. The RIST method uses variable glucose infusion rates to maintain the blood glucose level constant. The amount of glucose required to be administered in order to maintain the glycemic baseline is the index of insulin sensitivity and is referred to as the RIST index. The RIST index produced by this procedure consists of a HISS-dependent component and a HISS-independent component that can be readily differentiated by testing in the control fed state and then repeating the test after blockade of HISS release by any of a number of means including surgical denervation of the liver, blockade of hepatic muscarinic receptors, blockade of hepatic nitric oxide production, or blockade of hepatic cyclooxygenase. Eliminating HISS action by any of these procedures results in a reduction of the RIST index, in the fed state, of approximately 55%. That is, the glucose disposal effect that has been previously attributed to the direct action of insulin on a variety of tissues is actually mediated to a large extent by a hepatic insulin sensitizing process that has previously been unrecognized. This area has recently been reviewed (Lautt, 1999; Lautt, 2003). Blockade of HISS release results in HDIR. If HDIR is produced physiologically in response to fasting, these interventions do not produce any further decrease in insulin action.
HDIR is a normal and essential response to fasting. Insulin release occurs even in the fasted state and performs a number of growth regulating functions. Insulin is released in a pulsatile manner throughout the day with only approximately 50% of insulin release being regulated by food ingestion (Beyer et al., 1990). In the fasting state, it would be disadvantageous for insulin to cause a massive shifting of glucose from blood to skeletal muscle glycogen stores. The glucose disposal action in response to an injection of insulin decreases progressively to insignificance by 24 hours of fasting. This decrease in response to insulin represents a physiologically adjusted decrease in the HISS-dependent component as demonstrated by the observation that the HISS-independent (post-atropine or post-hepatic denervation) component of insulin action is similar in fed and 24-hour fasted rats.
In the immediate postprandial state, approximately 55% of the total glucose disposal effect of a bolus administration of insulin over a wide physiological range (5-100 mu/kg) is accounted for by HISS. By 18 hours of fasting, Sprague Dawley rats show HISS-dependent insulin action that accounts for only 26% of total insulin action (Lautt et al., 2001). The proportion of insulin action accounted for by HISS action remaining after 18 hours of fasting in cats is 35% (Xie & Lautt, 1995) and 25% in dogs (Moore et al., 2002). HISS action in rabbits accounts for approximately 44% of insulin action although the time since feeding was not stated (Porszasz et al., 2002). Fasting induces a 45% reduction in insulin action in mice (Latour & Chan, 2002). Preliminary results indicate that 62% of insulin action in the fed state is accounted for by HISS action in humans. This physiological regulation of HDIR is an appropriate response to fasting and, as such HDIR is a useful physiological state.
While HDIR is a useful physiological state in the fasted condition, failure to release HISS and the resultant HDIR in the fed state is suggested to account for the major metabolic disturbance seen in type 2 diabetes and many other conditions of insulin resistance. According to this model, post-meal nutrient processing normally results in approximately 80% of the glucose absorbed from a meal being stored in the large skeletal muscle mass of the body. Although HISS is released from the liver, it selectively stimulates glucose uptake into glycogen stores in skeletal muscle. Lack of HISS action results in a greatly impaired glucose disposal effect of insulin thus resulting in postprandial hyperglycemia. Additional insulin is released in response to the elevated glucose thus accounting for postprandial hyperinsulinemia in the type 2 diabetic. Insulin stimulates glucose uptake into adipose tissue and into the limited stores of the liver. When the glycogen stores in the liver are saturated, the remaining glucose is converted to lipid thus accounting for postprandial hyperlipidemia in the type 2 diabetic. The biochemical effects of hyperglycemia including the generation of free radicals has been suggested to account for the major non-metabolic pathologies common to diabetics including endothelial cell dysfunction, deposition of atherosclerotic plaques, blindness, renal failure, nerve damage, stroke, and hind limb amputation (Brownlee, 2001). HDIR has been shown to occur in chronic liver disease, fetal alcohol exposed adults, obesity, sucrose fed rats, hypertension, pregnancy and trauma.
The present inventors propose that HDIR is the main cause for type 2 diabetes, impaired glucose tolerance, impaired fasting glucose, hyperinsulinemia, hyperlipidemia, obesity, postprandial hyperglycemia and other insulin resistant states. For patients suffering from these disorders, the only approved form of treatment currently available is insulin and certain oral medications. The oral drugs fall into five main classes: sulfonylureas, biguanides, alpha-glucosidase inhibitors, meglitinide agents and thiazolidinedione agents.
These medications only achieve the best results when combined with a restricted diet and regular exercise. However, even then the treatment is not successful for all patients. Blood glucose levels drop but many never see a decrease to levels that are within the normal range and oral medications are known to spontaneously stop working for unknown reasons. In addition, success rate for individuals who have had type 2 diabetes for more than 10 years is very low. Oral medications are usually successful for the first three years of treatment, but at this point 50% of people with type 2 diabetes need additional therapy. After 9 years, 75% of people need combination treatment to keep their diabetes under control (Turner et al., JAMA 281:2005-2012, 1999)
Combination therapy comprising of two diabetes medications is prescribed in some cases when the single therapy proves to be ineffective. However, the combination of oral therapies is limited, and only certain combinations can be given simultaneously. Sulfonylureas and meglitinide agents can be administered together, but can cause hypoglycemia. Biguanide agents and thiazolidinedione agents cannot be taken with insulin secreting agents and acarbose, a commonly prescribed alpha glucosidase inhibitor cannot be combined with any other anti diabetic agent. The most common result of these combinations is hypoglycemia and weight gain.
Thus, there is a need for more effective and safer combination therapies for the treatment of diabetes and other insulin resistant states. Also, there is a need for treatments that address the specific mechanism involved in post-prandial hyperglycemia, that is, HDIR.